This invention relates to electrochemical apparatus and processes and, more particularly but not exclusively, is concerned with electrochemical apparatus employing particulate cathodes and with electrodeposition processes carried out using such electrochemical apparatus.
In general, electrochemical processes may be considered as being either cathodic processes or anodic processes depending on the electrode at which the economically important reaction occurs. Most cathodic processes involve either electrodeposition of a metal or electrolytic reduction of a constituent of the electrolyte in the presence of hydrogen formed at the cathode; in the former class of cathodic process are electroplating, electrorefining and electrowinning and in the latter class are the reduction of organic compounds and the production of caustic soda. Most anodic processes involve either the discharge of anions from solution at an essentially stable anode or the dissolution of the anode itself; in the former class of anodic process are processes for the production of chlorine and oxygen and in the latter class are processes for recovering valuable metal from scrap and the refining or purification of metals. Details of conventional industrical electrochemical processes are given inter ali in the book entitled "Industrial Electrochemical Processes" edited by A. Kuhn and published in 1971 by the Elsevier Publishing Company.
A number of electrochemical processes make use of so-called bipolar electrodes. These bipolar electrodes have one face at which a cathode reaction occurs and another face at which an anode reaction occurs. Bipolar electrodes have found application inter alia in electroplating processes where metal is electrodeposited on the cathodic face of the bipolar electrode but passes into solution at the anoidic face thereof. There are some electrochemical processes where the electrodes maintain substantially constant dimensions as the cell reaction proceeds, e.g. where both the anode and cathode reactions comprise the evolution of a gas at the surface of the respective electrode, in which bipolar electrodes have been used as separators to separate adjacent cells in an assembly of electrochemical cells arranged in electrical series. However, in electrochemical processes where the dimensions of one of the electrodes changes as the cell reaction proceeds, e.g. when there is electrodeposition of metallic ions onto the cathode, the use of bipolar electrodes to separate adjacent cells has not been practicable because of the need to periodically remove from the cell, and replace, electrodes whose dimensions change as the cell reaction proceeds.
There have recently been described various forms of electrochemical apparatus which essentially comprise an electrochemical cell having an ion-permeable diaphragm disposed between the electrodes of the cell and in which the cathode is a particulate electrode comprising a plurality of electroconductive particles on which, for example, a metal can be electrodeposited; one such apparatus is described, for example, in Belgian Pat. Spec. No. 818,453, this apparatus including an electrode system, suitable for use with an anodic counter electrode to perform an electrochemical process, which electrode system comprises a particulate cathode, a current conductor (frequently known as a current feeder), a vessel which contains the particle cathode and current conductor and has one ion-permeable wall at least a part of which is inclined towards and overlies the particulate electrode, and means for flowing a fluid medium through the vessel in contact with the particulate cathode. Other examples of particulate electrodes are disclosed in, for example, British Pat. Specification No. 1,194,181, U.S. Pat. Spec. Nos. 3,180,810, 3,527,617 and 3,551,207 and French Pat. Specification No. 1,500,269. Electrochemical apparatus employing particulate cathodes can be used, inter alia, in processes for electrowinning metals. Thus, Belgian Pat. Specification No. 818,453 discloses a process for electrowinning a metal from an electrolyte comprising an aqueous solution of one or more salts of a metal in which process the electrolyte is passed through a cathode compartment of an electrochemical cell, the cathode compartment comprising an electrode system of the type defined above, whilst small electroconductive particles are fed to the cathode compartment, wherein they form part of the particulate cathode, and enlarged particles on which metal has been electrodeposited are extracted from the cathode compartment, the distribution of the particles of the particulate cathode in the cathode compartment being controlled during the process in a manner such that substantially all the particles are circulated between first and second regions formed within the cathode compartment, the first region being adjacent to the ion-permeable wall, within which first region substantially all the particles are for a large proportion of the time they spend in the first region, out of contact with each other, and the second region being spaced from the ionpermeable wall, within which second region substantially all the particles are, for a large proportion of the time they spend in the second region, in contact with other particles.